Mixed plastic waste usually contains, apart from a large number of different types of plastic, further valuable materials in part firmly connected to one another, as well as impurities.
Although many individual process steps for the recycling of such plastic waste are known in the prior art, such as for example the mechanical milling of the plastic waste into a ground material or the separation of two components by means of sink-or-float separation or by means of the new near-infrared separation technique, the known methods are bound up with the drawback that a high separation selectivity, which is a pre-requisite for obtaining plastics of high and constant quality, cannot be achieved for such mixed plastic wastes.
For sustained recycling, it is necessary to separate the individual materials in a manner that is as grade-pure as possible, in order that they may successfully replace new goods free from impurities in markets segments and can therefore contribute significantly both to the competitiveness of the end product as well as to the preservation of resources.
If the mixed plastic waste is, for example, rejects from liquid cartons for drinks such as milk, fruit juice or suchlike, which upon separation of the paper fibres become available for cardboard production in the paper-mill, then films, laminated films, hard plastic parts and residual paper fibres are contained in this mixed plastic. The films predominantly comprise low-density polyethylene (LDPE), the laminated films comprise aluminium foil and LDPE film, and the hard plastics from the lips and closure caps comprise high-density polyethylene (HDPE) and to a lesser extent polypropylene (PP). Moreover, such drinks cartons also contain a quantitatively negligible proportion of film strips of PA or PET, which are used to seal the line joint.
It is true that many individual process steps are known which could be used in the recycling of such a mixed plastic, but altogether they have not hitherto led to a situation where the quantitatively relevant plastic fractions of the drinks cartons or the rejects are separated in any one process into their individual valuable material fractions in a way such that they can be recycled as pure materials.
The current practice in the recycling of the predominant quantity of drinks carton rejects is their use for energy purposes in cement kilns and/or refuse incinerators. Out of the approximately 106 MJ/kg of accumulated energy content contained on a statistical average in the drinks carton rejects, just around 23 MJ/kg of reject is utilised in energy terms in the case of their recycling in the cement kiln, despite its very good energy utilisation efficiency. In the utilisation of drinks carton rejects for purposes of energy, approximately 83 MJ/kg of the accumulated energy contents contained therein is therefore lost, which corresponds to approximately 78%.
As the oldest practised material recycling process for these rejects, mention may be made of the so-called Corenso process (Finland). Apart from the utilisation of the plastic components of the drinks cartons for energy, it also includes at least a material recycling operation for the aluminium fractions. According to VDI Nachrichten dated Oct. 6, 2005, the rejects undergo pyrolysis in this process in a gas reactor at temperatures above 400° C. The polyethylene begins to gasify from 400° C. (pyrolysis gas), the aluminium being maintained in solid form at this temperature. During its combustion, the combustible pyrolysis gas then delivers electrical and thermal energy. The aluminium occurs as an agglomerate and is necessarily contaminated by the gasified impurities. After an operation of the installation lasting over 10 years, it can be stated that this technique has not been installed once again worldwide, although there is worldwide interest in recycling the rejects of drinks cartons in terms of material. In contrast, it has to be stated that the Corenso installation is no longer producing in 2009.
A new process for reject recycling was commissioned in a pilot plant close to São Paulo in Brazil (“Mit dem Ziegelstein gegen den Klimawandel”, Journal Getränkeindustrie November 2007, pages 10 and following). In this process, the rejects are treated with a plasma beam at over 1100° C. The plastics are decomposed into a low-grade ethylene glycol on account of the large number of types of plastic present in the rejects, said ethylene glycol having to be reprocessed in a refinery. The aluminium of the rejects leaves the process in recyclable bar form. Even though the polyolefins of the rejects are recovered as low-grade ethylene glycol, a very large proportion of the accumulated energy consumed for the production of the new goods therefrom is lost on account of the very high energy expenditure of this process on the one hand and on account of the low energy content of the low-grade glycol on the other hand.